Our work investigates the mechanism of mixed leukocyte tumor interaction in the SJL/J mice. These mice develop spontaneous reticulum cell sarcoma (RCS) and die from the tumor. The failure of the immune response to reject the tumor may be due to inability of the host to recognize the tumor antigens. We are proposing to examine host recognition of the tumor by assessing in vitro responses of SJL/J lymphoid cells to syngeneic RCS in both the proliferative response and the cytotoxic effector response. The antigens involved in the activation of the responder cells will be examined by the use of various antisera directed against H-2 and non-H-2 tumor associated antigens on the tumor. The stimulating tumor cell will be derived from spontaneous, transplantable and culture lines, and these will be compared in the in vitro responses. A correlation between in vitro responses and tumor development will be established.